🛠 Lire un capteur I2C (température/humidité) sur le bus 1

Objectives

Enable the I2C bus on the BCM2835 (only one hardware bus is exposed on P1: I2C0 on GPIO0/1), scan the bus with i2cdetect, and read a digital sensor in Python. Early revisions do not have wired I2C pull-up resistors — a topic discussed on official forums — so we add external 4.7 kΩ pull-ups for safety.

Hardware

• Raspberry Pi Model B Rev 1, legacy 32-bit image (Buster)
• 3.3 V I2C sensor (e.g., BMP180, address 0x77)
• 2 pull-up resistors 4.7 kΩ to 3.3 V (SDA and SCL)
• sudo apt install -y i2c-tools python3-smbus
• Reminders: GPIO0 = pin 3 (SDA0), GPIO1 = pin 5 (SCL0), 3.3 V = pin 1, GND = pin 6.

Complete Code

# 1) Activer I2C : sudo raspi-config > Interfacing > I2C
#    ou manuellement dans /boot/config.txt :
#    dtparam=i2c_arm=on
sudo reboot

# 2) Scanner le bus (le capteur doit apparaître, ici 0x77)
sudo i2cdetect -y 1

#!/usr/bin/env python3
"""Lecture brute d'un registre capteur via SMBus (adaptez l'adresse)."""
import smbus

BUS = smbus.SMBus(1)       # bus I2C1 cote linux (peripherique i2c-bcm2708)
ADDR = 0x77                # sortie de i2cdetect -y 1

val = BUS.read_byte_data(ADDR, 0x10)   # exemple de registre
print("registre 0x10 =", hex(val))

Expected Verification

i2cdetect -y 1 displays a grid with the sensor's address (never 00, nor reserved addresses). The Python script prints a non-zero value that changes if you warm the sensor between your fingers.

Common Errors

• Forgetting dtparam=i2c_arm=on and rebooting: /dev/i2c-1 does not exist.
• Bus 0 vs bus 1: on the BCM2835, the bus exposed on P1 is bus 1 from Linux's perspective; i2cdetect -y 0 often returns empty.
• 5 V sensor on a 3.3 V bus: choose a 3.3 V module or a level shifter.
• Missing pull-ups: the bus remains stuck in the low state, i2cdetect displays nothing or UU.
• Permission denied on /dev/i2c-1: add the user to the i2c group or run with sudo.